The invention relates to a display device comprising an electro-optical material between two substrates, at least one of which is transparent, and a first substrate is provided with at least one picture electrode at the location of a pixel, each pixel being coupled to a row electrode and a column electrode, control means comprising first drive means for applying a selection signal to the row electrodes and second drive means for applying a data signal to the column electrodes.
Display devices of this type are used in, for example, televisions, monitors, laptop computers, etc.
Usually, the second substrate comprises one or more counter electrodes but this is not strictly necessary as in, for example, the case of xe2x80x9cin-plane switchingxe2x80x9d (IPS).
Display devices of the type described above are generally known and are usually driven by means of alternating voltages across the pixels (AC driving) so as to prevent degeneration of the liquid crystal materials. Nevertheless, it has been found that, due to different causes, a parasitic DC component may be produced across the layer of liquid crystal material. This is particularly the case when the pixels have an asymmetrical structure, as is the case, for example, in reflective display devices (in which the display device comprises a reflector, or the picture electrodes on one of the substrates are reflecting).
Said DC component affects the drive of a pixel differently for opposite polarities in successive frame times. When the absolute voltages across a pixel in successive frame times (at the same data) differ, this will give rise to flicker at half the frequency of the frame frequency used (generally 50 or 60 Hz) which is very clearly visible in the image.
It is, inter alia, an object of the invention to provide a display device of the type described above, in which the above-mentioned drawbacks are at least partly obviated.
To this end, a display device according to the invention is characterized in that the display device comprises a measuring element, and the control means comprise means for applying a voltage to the measuring element during a selection period and for measuring the variation of the voltage across the measuring element after the selection period, and for adapting, dependent on the measured voltage variation, at least one of the control voltages of the display device generated by the control means.
The control voltage to be adapted is, for example, a voltage of a line selection signal, a data signal, a reference voltage of the display device (for example, a reset voltage, or the voltage across a control electrode) or, when the second substrate comprises at least one counter electrode, the voltage of a signal across the counter electrode.
The invention is applicable to display devices of the passive and active type.
For example, a line selection period of the display device is chosen for the selection period.
It is found that the parasitic DC component may give rise to differences in the (measured) voltage variation in the different (positive and negative) frame periods. When driving pixels, this difference leads to said flicker. By comparing the variation of the voltage after two consecutive selection periods (or in two consecutive frame periods) with each other and by adapting one of the control voltages, dependent on the measured result, the flicker is reduced considerably.
In a first implementation (active display device), each pixel is coupled to the row electrode or the column electrode via a switching element. In this case, the measuring element may be constituted by, for example, a row of pixels, but preferably the display device is provided with a row of extra (dummy) pixels.
The measured voltage difference can be compared with a variation stored in advance in the control means, for example when the display device is adjusted in advance.
A preferred embodiment of a display device according to the invention is, however, characterized in that the control means comprise means for reversing the sign of the polarity of the voltage across the measuring element and for measuring the difference between the voltage directly after the selection period and the voltage just before a subsequent selection period, and means for adapting the control voltage of the display device in such a way that the absolute value of the voltage difference for both polarities is substantially the same. A small number of measurements may then be sufficient. In this case, measurement and correction take place continuously. In another implementation, measurement and correction are performed once, for example when switching on the display device, or periodically.
When used in passive displays, for example, a measuring element (outside the actual display section) is used which is directly driven from the control means.